The invention relates to plasma ion sources, and more particularly electron cyclotron resonance (ECR) ion sources.
An ion source is a plasma generator from which beams of ions can be extracted. Multi-cusp ion sources have an arrangement of magnets that form magnetic cusp fields to contain the plasma. Plasma can be generated in a multi-cusp ion source by DC discharge or RF induction discharge. In an RF driven source, an induction coil or antenna placed into the ion source chamber is used for the discharge. The principles of plasma ion sources are well known in the art. Conventional multicusp ion sources are illustrated by U.S. Pat. Nos. 4,793,961; 4,447,732; 5,198,677; 6,094,012, which are herein incorporated by reference.
The electron cyclotron resonance (ECR) ion source is a type of source in which ions are obtained through ionization of a gas by electrons that are accelerated by electron cyclotron resonance. ECR sources are typically used to provide beams of multiply charged ions. ECR results from the interaction of a static magnetic field with an injected high frequency electromagnetic field, i.e. ECR heating in which electrons are circulating at the selected microwave frequency around magnetic field lines in resonance. A conventional ECR source is illustrated by U.S. Pat. No. 5,256,938, which is herein incorporated by reference.
A problem with conventional ECR sources is its complicated electromagnetic and permanent magnet structure. The plasma confining magnetic field structure typically is a superposition of a solenoid field and a multipolar field, similar to a cusp field, producing a “magnetic bottle.” The magnetic field has a minimum in the center region of the source and increases towards all directions. Between the source center and the source wall there is a closed equi-magnetic surface (a thin layer), called the ECR resonance layer, where the ECR frequency for the electrons equals the microwave frequency. The electrons are bounced back and forth in the magnetic mirror, gaining energy every time they pass the resonance layer, heating the electrons up to hundreds of keV. The ions are then trapped by the space charge of the electrons.
ECR sources normally operate at very low gas pressure to prevent voltage breakdown. Therefore, the plasma density and extractable current are low. It is also difficult to control charge state and output current independently. Further, ECR ion sources generally have higher ion beam emittance than other types of ion sources. Accordingly it is desirable to produce an ECR type ion source that reduces or eliminates these problems.
EUV lithography using 13 nm EUV radiation is a prime candidate for a sub-100 nm lithography tool. The 13 nm EUV radiation is generated by either a laser-produced or a plasma discharge source. The laser-produced EUV source is very low in power efficiency, expensive and large in size. The plasma discharge source is compact and economic but produces a low level of EUV radiation. The semiconductor industry is seeking a low cost, compact, intense, efficient EUV source.
The most common method of producing 13 nm EUV radiation is to employ Xe+10 ions. If one can populate a plasma with a high concentration of Xe+10 and the plasma density is high, then the intensity of the 13 nm radiation will meet the requirements of the next generation lithography tool.